Cnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic environments:
they are predominantly marine species. Their distinguishing feature is cnidocytes, specialized cells that they use
mainly for capturing prey.Most cnidarians prey on organisms ranging in size from plankton to animals several times
larger than themselves, but many obtain much of their nutrition from dinoflagellates, and a few are parasites.
Not all cnidarians reproduce sexually, with many species having complex life cycles of asexual polyp stages and
sexual medusa.While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other
cnidarians can be found at great depths, in polar regions, and in freshwater. Fossil cnidarians have been found in rocks
formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490
million years ago and diversified a few million years later.
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